If you read the instructions, you will discover https://haveibeenpwned.com/Passwords which will let you enter a password and securely check if it has been published in a breach.

If it has, it is either a simple password that multiple people are using, or a complex secure password that can make you pretty confident it is your password that has been published.

1Password just does the same thing for all of your passwords - it doesn’t check against your account name either. That information isn’t stored so they can’t become a new source of breached accounts (as explained at the site).